The present invention relates to a color image processing method and apparatus for converting three color signals read by an image input device to color image recording signals of a plurality of colors including black. More specifically, the invention relates to a color image processing apparatus for determining a black signal and what is called an under color removal quantity in the image recording signals.
In the printing technologies, a color original image is usually recorded or reproduced by 4-color printing. That is, the printing is performed using color separation printing patterns for printing inks of yellow, magenta, cyan and black. This is due to the fact that three-color printing of yellow, magenta and cyan cannot produce reproduction images having sufficient contrast performance because, for instance, the inks do not have an ideal color development characteristic.
In some cases of the 4-color printing, the printing inks of yellow, magenta and cyan are subjected to what is called 100% under color removal, in which an image is reproduced using black ink and two of yellow, magenta and cyan inks. This technique has advantages that the color reproduction range is broadened in low lightness portions and gray stability is kept high in high lightness portions. Further, the under color removal serves to reduce consumption amounts of expensive color inks, to thereby lower the running costs.
Although the various advantages can be obtained by performing the under color removal in the 4-color printing, it is very difficult to determine the under color removal quantity and the black quantity in accordance with the input image signals. In general, since black ink has a higher contrast than the other inks, it likely enhances roughness of an image and therefore it is not appropriate to include a large quantity of black ink in human skin portions in an image. Further, the black quantity in character images needs to be larger than in photograph image, to increase the sharpness of characters.
Various techniques of determining the under color removal quantity and the black quantity in a printing color scanner are disclosed in Japanese Patent Application Unexamined Publication Nos. Sho. 57-173838, Sho. 58-190951, Sho. 58-211757, etc.
In the publication Sho. 57-173838, the under color removal is performed separately in chromatic color areas and in achromatic color areas. Achromatic color portions are reproduced only by black ink, and the black quantity is gradually changed in a transition area from an achromatic area to a chromatic area.
The publications Sho. 58-190951 and Sho. 58-211757 disclose techniques for determining the black quantity and the under color removal quantity in accordance with gradation levels of an input image. According to this technique, complete achromatic printing is performed up to a gray level corresponding to the over color of black ink used, and quantities of color inks are increased continuously in shadow areas having a gray level higher than the above level. That is, gray portions having a density within a range reproducible by black ink is reproduced only by black ink, and gray portions whose density exceeds the above range are reproduced using the same quantities of the other three color inks.
Further, techniques of determining the under color removal quantity and the black quantity in digital color recording technologies such as ink jet printing, thermal transfer recording and laser color xerography are disclosed in Japanese Patent Application Unexamined Publication Nos. Sho. 59-161981, Sho. 59-163973, etc. The publication Sho. 59-161981 describes an under color removal technique in which the black quantity is obtained by multiplying the minimum value of three color signals of yellow, magenta and cyan by a certain constant and subtracting the black quantity from the respective color signals. In the publication Sho. 59-163973, two color inks to be combined with black ink are determined from spectral reflectances of a plurality of inks, and the recording is performed such that the color inks and the black ink do not overlap each other. This technique can determine the black quantity and the under color removal quantity by a simple calculation.
Further, Japanese Patent Application Unexamined Publication No. Hei. 4-31859 discloses a color image processing method for determining a dot area ratio that is a parameter used for reproducing an input target color by yellow, magenta cyan and black. In this method, a maximum black area ratio is first determined, and then multiplied by a black ratio that has been determined only from a lightness signal, to determine a final dot area ratio of black. Then, dot area ratios of yellow, magenta and cyan are determined with the final dot area ratio fixed. That is, as shown in FIG. 6 of this publication, the black ratio is reduced for portions having a large lightness value and is increased for portions having a small lightness value. As a result, the target color can be reproduced more faithfully in areas having a low lightness, i.e., in dark areas.
However, in the technique of the publication Sho. 57-173838, many adjustment coefficients are needed in determining the black quantity and the under color removal quantity. Since the coefficients can be determined only empirically, the difficulty in determining the black quantity and the under color removal quantity still remains unsolved.
Since the publications Sho. 58-190951 and Sho. 58-211757 address only the processing on gray portions, a color quasi-outline, i.e., a chroma gap may occur in a transition area from an achromatic area to a chromatic area, i.e., in an image in which the chroma gradually varies (such as a usual color picture/pattern).
The technique of the publication Sho. 59-161981, which is generally called "constant ratio under color removal/addition," has a problem that it cannot perform correct color reproduction. The reasons are described, for instance, in "Considerations (I) on black inclusion in printing," Proceedings of 1st Joint Conference on Color Technologies, four optics societies, 1984, pp. 1-7.
In the publication Sho. 59-163973, since the calculation is based on the principles of the average additive mixture, it cannot perform correct color reproduction in actual recording. This problem originates from the light penetration and diffusion into a sheet, as described, for instance, in J.A.C. Yule, "Principles of color reproduction," Publication department of printing society, 1971, pp. 247-248.
Further, in the publication Hei. 4-31859, the black ratio is determined only based on the lightness signal, and no consideration is made of accommodating a variation of chroma. If the technique of this publication is used without modification in reproducing an image having a chroma variation, characters could not be expressed clearly with a small black ratio in bright areas and transition from a chromatic area to an achromatic area would become somewhat unnatural in dark areas because of a sudden increase of the black ratio. Specifically, satisfactory reproduction performance could not be obtained for an image in which pictures and characters are mixed.